1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method and system for improving signalling link capacity in a cellular communications system.
2. Description of Related Art
An important signalling system that has been optimized for use in digital telecommunications networks is the Consulting Committee for International Telegraph and Telephone (CCITT)-recommended Signalling System No. 7 (SS7). The SS7 protocol uses a layered architecture, wherein different layers use services from lower layers. The lower layers can be characterized as the SS7 infrastructure, which includes a Message Transfer Part (MTP), Signalling Connection Control Part (SCCP), and Transaction Capabilities Application Part (TCAP). Higher level protocols, such as the Mobile Application Part (MAP) and ISDN User Part (ISUP), are user applications of the SS7 infrastructure. As such, the MTP conveys signalling messages between the different user parts or application parts in the signalling network, and the SCCP supplements the message-carrying capabilities of the MTP.
In digital cellular communications systems, such as, for example, the Personal Digital Cellular (PDC) System in Japan, the MAP provides mobile-specific signalling capabilities required for the exchange of information between nodes in the PDC network. In this regard, the MAP uses the services of the MTP, SCCP and TCAP. As such, the basis for SS7 signalling in the PDC is a three-level MTP. The level-1 MTP performs signalling data link functions which can be implemented in two similar but oppositely-directed data channels to provide full-duplex signalling. The level-2 MTP performs signalling link functions which are used to transfer variable length signalling messages between two signalling points. The level-3 MTP performs signalling network functions which are used to perform signal unit management and handling tasks.
In digital cellular networks (e.g., in the PDC System), the mobile station registration process involves the transfer of registration information in MTP messages over signalling links between a Mobile Services Switching Center (MSC) and the Home Location Register (HLR). However, in the PDC System, the capacity or throughput of the MTP links between the signalling points (e.g., nodes) is limited (e.g., 4.8 or 48 kbit/sec). Consequently, the number of mobile registrations that can occur per unit of time is also limited. As such, a maximum of 16 signalling links can be installed between an MSC and HLR in a conventional PDC network. However, the MTP signalling links (and associated hardware) between these nodes are relatively costly to install. Therefore, in order to reduce equipment and operating costs, PDC network operators typically limit the number of MTP links between these nodes to 4.
A significant problem that exists because of such limited MTP signalling link capacity in digital cellular networks is illustrated by the restart process in a PDC network. For example, when a restart procedure is initiated in a particular MSC, the MTP signalling links between the MSC and HLR can become overloaded, although the nodes themselves are only loaded to about 30% of their capacity. This signalling link overloading can last anywhere between 10 to 50 minutes depending on the size of the particular network involved. The conventional solution to this link overload problem is to restrict the number of mobile stations that can access the network during a particular period of time. This solution provides a form of load protection for these links, because the links would otherwise be blocked due to congestion and treated by the network as xe2x80x9cout of servicexe2x80x9d.
However, a significant problem still remains because in return for a network operator providing such load protection for a signalling link, the throughput of the link may be reduced to about 50% of its theoretical maximum capacity. Nevertheless, as described in detail below, the present invention successfully resolves these and other related problems.
In accordance with a preferred embodiment of the present invention, a method is provided for increasing MTP signalling link capacity between nodes in a digital cellular communications system, wherein the MTP message to be conveyed over the link is compressed. The compressed MTP message received at the other side of the link is then decompressed. Depending upon the amount of compression used per message, the capacity or throughput of the link can be increased significantly. The present method can also be used for compressing other signalling messages to improve link capacity.
An important technical advantage of the present invention is that the capacity of the signalling links between nodes in a digital cellular communications system can be increased up to 100% of their maximum capacity.
Another important technical advantage of the present invention is that a digital cellular network operator can reduce the duration of a signalling link overload that occurs after restarting an MSC.
Still another important technical advantage of the present invention is that the capacity of nodes and signalling links in a digital cellular communications system will be optimized and their throughput will be maximized.
Yet another important technical advantage of the present invention is that mobile station registration restrictions can be minimized in a digital cellular communications system.